The present invention relates to coated abrasives and particularly to coated abrasives comprising specific forms of abrasive particles.
Coated abrasives of the conventional kind comprise a backing material and, adhered to the backing material, a maker coat which serves to anchor abrasive grits deposited on the maker coat while it is still in an uncured state. The maker coat is the initial bond between the grit and the backing but the primary means by which the grit is held in position while the coated abrasive is actually in use is a size coat that is applied over the tops of the abrasive grits.
With conventional randomly shaped grits the depth of the size coat and therefore the volume and cost of such coats is relatively modest. If however the grits become more elongated and are aligned preferentially perpendicular to the backing surface, the conventional amount of size would be located adjacent an extremity of the grit with the balance projecting above. When used for grinding, a force is in effect applied to the grits in contact with the surface being ground in the direction of relative movement of the workpiece with respect to the grit. The longer the grit the greater the moment of that force and the more likely the grit is to fracture above the size level or even be torn out of the size layer altogether. One solution to this problem is to add a deeper size layer but this becomes expensive. Another is to incorporate filler materials to the size such that, with the same volume of binder component in the size, the actual volume of the size formulation is increased by the volume of filler added. This does however have the effect of reducing the strength of the bond between the grit and the binder such the grits might more easily torn out during grinding.
Nevertheless there is considerable advantage to be gained from providing elongated abrasive grains oriented perpendicular to the backing if these can be adequately anchored to the backing because, if they wear down in a controlled fashion and do not become dulled, the useful life of the coated abrasive can be significantly extended.
Elongated grits however have a potential problem in that when deposited on a conventional maker coat by the conventional electrostatic precipitation technique, (called the UP process), they stand on end being held only at their extremity by the maker coat. This posture is relatively easily disturbed by flexing the substrate or by passing the coated material through a roller nip or even by the use of a knife coater to apply the size coat. Therefore the maintenance of the orientation of the grits on the backing during further manufacturing stages becomes a matter of some difficulty.
Abrasive grits with a highly elongated shape are described for example in U.S. Pat. Nos. 5,009,676; 5,103,598; 5,090968; and 5,244,477. Such grits usually have a uniform cross-sectional shape and have an aspect ratio, (the ratio of the longest dimension to the greatest dimension perpendicular to the longest dimension), of greater than 1 and more usually at least 2:1 and frequently from 4:1 to 25:1 or more. Such grits are typically made from a sol-gel alumina and more suitably from a seeded sol-gel alumina. These aluminas comprise sub-micron crystals of alpha alumina and during grinding these break off in controlled fashion exposing fresh cutting surfaces. Such grits therefore show excellent grinding performance.
It is therefore highly desirable to develop ways in which these sol-gel alumina grits can be used to the greatest advantage. This in practice resolves itself into ways of producing an abrasive product in which elongated abrasive grits are oriented so as to present grit ends at the cutting surface of the coated abrasive.
The present invention provides such a product and a process for obtaining such a product. The grits are held very strongly and the orientation can be made essentially constant.